During the process of developing a circuit design, the behavior of the design is often simulated to verify correct behavior prior to physical implementation of the circuit. One source of behavioral discrepancy is the distributed effects of power delivery in a realized circuit design. As current passes through a component, a voltage perturbation results in the power grid network. The voltage perturbation can affect timing of the circuit because a perturbed supply voltage modifies the delay of components such as logic gates or interconnects. If the modified delays are not accounted for accordingly, the results of chip timing analyses can be erroneous. Other effects resulting from non-linear electrical characteristics of the components create additional fluctuations in voltage. These effects were generally ignored in older technologies because of relative slow chip speed and low integration density. However, as speed and density of circuits increase, the unintended effects caused by the parasitic electrical characteristics of components have become significant. Among other effects, inductance of connections to the power grid in combination with power grid capacitance can resonate when perturbed. It is desirable to simulate these effects to provide an accurate analysis of power grid performance.
Historically it has been difficult to derive a passive power grid model of a die due to its complexity. There has also been a lack of tools to address this task, since in older technology the distributed effects of power delivery could be ignored. However, over the last few years, silicon manufacturers and electronic design automation vendors have refined techniques to derive passive power grid models based on actual metal layer routing of a silicon die, and at this point are able to create power grid models with a useful level of accuracy in a reasonable amount of time.
Currently, however, the accuracy provided by running noise simulations of these power grid models is limited by the lack of a stimulus model of a sufficient level of detail or granularity. Simulation of the whole device at the transistor or gate level would provide the granularity necessary to account for the distributed aspects of the power grid and feedback, yet such a simulation is impractically large. Some level of abstraction is required to both provide a desired level of detail and complete simulation in a feasible amount of time with an available amount of computing power.
Some previous techniques for simulating non-ideal voltage in the power grid network performed a static or DC analysis. In these simulations, an average current is used to represent the actual current, and AC or time-variant characteristics are ignored. However, actual components of a circuit draw time-varying currents from the power supply network in performing state switching activities. These time varying currents give rise to time-varying voltage on the power network. Therefore, the static simulation does not provide an accurate representation of performance as it does not provide any transient voltage information. The results provided by the DC approach are not adequately representative as it cannot account for many physical effects such as those arising from the placement of de-coupling capacitors or the speed of the transitions. Further, the capacitance and inductance of the power grid does not impact the results of the static model, since only the resistance matters in the static DC simulation. In addition, the timing of the transitions of the components has no effect on the static simulation even though multiple components switching simultaneously cause significantly different results from non-simultaneous switching in an actual circuit.
Another approach is to group similar components together in an AC simulation to produce a single stimulus on the power grid network that is an average of the group. While gross approximations of current transients can be derived from measurements of voltage noise measured at some point in a passive power grid model, any distributed aspects of the power grid model will not be correctly simulated. The current stimulus and its effect on power grid voltages is distributed, however information about this distributed nature is not captured by the measurement of voltage at a single point, which often is not even on the die.
One or more embodiments of the present invention may address one or more of the above issues.